The present invention relates to a sensor node technique and more particularly to a sensor node for detecting impacts.
A sensor node senses information unevenly distributed in its environment at designated intervals of time, and transmits the sensed information values by wireless communication. On account of its intrinsic function to sense and transmit by wireless communication information unevenly distributed in its environment, a sensor node has to be able to operate on a battery for a long period.
Conventional devices for detecting impacts arising at irregular intervals of time for use in sensor nodes detect such impacts with a mechanical switch or a switch using electric power generated by electromagnetic inductance or a piezoelectric element, or through the measurement of variations in acceleration with an acceleration sensor which is kept operating all the time.
A conventional impact detecting sensor using a piezoelectric element utilizes the trend of the vibration, which accompanies the operation of the device, to increase when an abnormality arises, and detects a signal which is generated when the. vibration has surpassed a certain amplitude (see, for instance, Japanese Patent Application Laid-Open Nos. 8-145783 and 9-264778).
In another conventional sensor, when the acceleration working on the piezoelectric element is at or above a certain level, a voltage is applied to the gate of a MOS-FET to turn it on and detect a signal, and the duration of its being kept on can be adjusted with a resistor (see, for instance, Japanese Patent Application Laid-Open No. 10-260202).